This invention relates to spacecraft such as geostationary communications satellites which include thrusters for insertion into orbit and which also include thrusters for stationkeeping.
Satellite communications are widely used. For example, geostationary satellites placed in fixed equatorial orbits can provide communication over a broad geographic area. At present, a large amount of television distribution among terrestrial broadcast stations is accomplished by satellite. A typical communications satellite may include 10 to 40 wideband transponders, each of which may be leased for as much as $1 million per annum. Polarization isolation allows each transponder to be used for two partially overlapping signals, which effectively doubles the number of transponders. Thus, an operating satellite generates a great deal of revenue. Considering that manufacture and launch of a satellite may cost $50-100 million, it is imperative to get as much useful life from each satellite as is possible.
In the past, gradual degradation of equipment, together with random failures, tended to be the limiting factors in satellite life. Improvements in the reliability and life of satellite components has resulted in satellites which continue to operate until the fuel required to maintain station is exhausted. Thus, the useful life of a satellite may be directly related to the amount of stationkeeping fuel which can be loaded and launched. Great efforts are expended in minimizing the weight of nonessential portions of the satellite so that the maximum amount of stationkeeping fuel may be loaded and launched.
In general, satellites may be launched by expendable boosters or by recoverable vehicles, such as a space shuttle. The manufacturer or operator of the launch vehicle, whether expendable or nonexpendable, specifies the weight of the satellite and the altitude at which it will be released. Very often, the satellite manufacturer provides a further booster, such as an apogee motor, to lift the vehicle from a low intermediate orbit at which the satellite is released to the desired orbit, as, for example, the 22,400 mile geostationary orbit. Thus, the satellite as it arrives at a low earth orbit includes an apogee motor for boost to the geostationary orbit together with some means for stationkeeping, which may include additional thrusters.
There are two general types of chemical thrusters; bipropellant and monopropellant. The bipropellant thruster uses a fuel and an oxidizer, as, for example, a monomethylhydrazine fuel (CH.sub.3 N.sub.2 H.sub.3) and nitrogen tetroxide (N.sub.2 O.sub.4) as an oxidizer. The bipropellant system provides more thrust per unit weight of propellant than a simple monopropellant system, i.e. it is more efficient in that it provides a greater velocity change per unit mass or weight of propellant (where the term propellant in this context includes both fuel and oxidizer). The monopropellant system includes a thruster having a catalyst which causes a chemical change when contacted by the monopropellant fuel, which in turn provides thrust. The simple monopropellant system is less efficient than the bipropellant system. The use of excess electrical energy to heat the combustion products can make low-thrust monopropellant engines as efficient as bipropellant engines. However, the amount of electrical energy necessary for large thrusters cannot be supplied, so for large-thrust engines, bipropellant engines continue to be more efficient.
Bipropellant fuel systems suffer from the disadvantage that the engine oxidizer-fuel mixture ratio is subject to errors which cannot be predicted, which may typically account for 3-5% of the total fuel load. Since the errors cannot ordinarily be predicted, the fuel and oxidizer tanks are preloaded with an amount of propellant predicted based upon taking into account the possibility of worst-case mixtures. Consequently, if the nominal conditions prevail, the fuel and oxidizer tanks would run dry simultaneously if the engine were run until it stopped for lack of propellant. However, as a result of the unpredictable errors in oxidizer-fuel mixture, it can be expected that one tank or the other will run dry first. Naturally, it is very desirable to optimize the system so as to fully utilize all of the fuel and oxidizer. Significant concerns exist regarding the chemical compatibility of oxidizer with typical materials used to contain the oxidizer over the 10-year satellite lifetime. Also, the products of combustion may be corrosive, especially if free oxidizer is released.
The monopropellant system has the advantage that all of the monopropellant can be used to provide a velocity change. Also, hydrazine monopropellant fuel is well characterized for long space missions, and the combustion products may be less corrosive than those of a bipropellant system. In order to take advantage of the efficiency of the bipropellant system in generating a velocity change and the full utilization achievable with the monopropellant system, dual-mode propulsion systems have been used. Such dual-mode systems include a relatively high-thrust bipropellant apogee engine and smaller monopropellant thrusters.
At launch, the tanks of satellites including such dual-mode propulsion systems are loaded with enough oxidizer to provide sufficient thrust for a predetermined velocity change (.DELTA.V) from the bipropellant apogee engine under worst-case engine mixture conditions. The weight is brought to the maximum allowable booster or shuttle launch weight with propellant.